Construction Fill a Problem for Landscapes

Q. We recently purchased our house and in the process of installing our backyard.  Currently we are digging trenches for irrigation lines and very quickly finding out that this so called "soil" is in fact construction infill. The "soil" is comprised of dusty dirt and filled with rocks. Our landscape plans comprise of a small turf patch for our 1 year old daughter and our 3 year old dog, perimeter landscaping and a 18" wall off our patio for a small herb garden. We are have a very small yard and the finished landscape area, where soil will be placed, is only 365 square feet.  What do you recommend that we use in our yard to amend the infill and turn it into useful soil? And how much will we need? 

Viragrow's organic topsoil

A. What you are describing is pretty common in most urban residential settings in planned communities. This construction “fill” is junk and worse than our native desert soil. This fill will be okay for desert "flooring" of your landscape where there are no plants but will need to be replaced or amended in areas where you are planting in the ground, establishing a lawn or a vegetable area. By the way, you do not want to put soil with pointed rocks back in the trenches where there are irrigation lines. The soil beneath irrigation lines should also be free of rocks.

Another option is some contouring and elevation changes of your landscape with decent quality "topsoil". I am consulting with a company in Las Vegas, Viragrow, which has a source of local "topsoil" that is decent and can be used for elevation changes and contouring at a fairly reasonable price. I don't know of another company in town that would have anything this good for that purpose. The others all use “reject sand”. Reject sand is horrible stuff.

Viragrow's washed silica sand

You could also use soil mixes for doing the same thing but that will be more money. If you do any elevation changes to your landscape make sure the water that collects in your landscape in low spots after heavy rains does not enter the home. This is a desert but rain, when it occurs, can come down in torrents. Residential yards are required to have positive drainage from the backyards to the city streets and ultimately into storm sewers. This is to prevent flooding inside the home from happening during the storm events so make sure you contour/elevation changes address this problem.

If you are not doing any elevation changes and planting directly into your existing fill, excavate to the following depths for planting areas: 

• flowerbeds and vegetable beds 12 to 15 inches, 
• small shrubs and groundcovers 12 to 15 inches, 
• larger trees and shrubs 18 to 24 inches, fruit trees 18 to 24 inches, 
• lawns 12 to 15 inches, 
• vines 12 to 15 inches

… Basically the larger the plant the deeper and wider you should excavate.

Garden Soil Mix, compost and silica sand

Holes in the soil for planting need to be deep enough for the roots of the plant going into the hole but much wider than the roots. Holes for planting woody plants should have a width 3 to 5 times their existing width. Remove rocks from the backfill going into the whole that are larger than a golf ball. If the soil removed from the hole is native desert soil, mix this soil 50-50 with 100% compost. Use this amended soil for planting. If the soil excavated from the whole is mostly rock then use a finished soil mix such as the Viragrow Garden Soil Mix for backfilling these holes. Viragrow Garden Soil Mix is a mix of silica sand and compost, not reject sand.

Flower beds and vegetable beds need to be excavated to the depth I mentioned above and filled with a garden soil mix if the soil cannot be amended with 100% compost. Like I said, if this is native desert soil and not fill you could amend this soil with 100% compost and it should be fine.

The amount you will need is a little bit tricky. You would need 12 yards to fill an area about 350 square feet to a depth of 12 inches. I'm not saying that you need to order this amount but this should give you some idea of your starting point and worst case scenario.

Order 100% or straight compost if you can mix it with native desert soil. If you cannot mix it then I would order Viragrow's Garden Soil Mix as a general mix for planting all of your ornamental plants, fruit trees and lawn. This soil mix can also be amended for vegetables. You can always add extra ingredients to this for your vegetable garden area or your flowerbeds if you need to.

Bob Morris, Consulting Horticulturist

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